This invention relates to electrically conductive polymer composite materials, and more particularly to methods for improving the electrical conductivity of such materials.
Normally electrically insulating polymers can be made electrically conductive via the addition of electrically conductive fillers, such as carbon fibers, carbon blacks, or metal fibers. In each case, sufficient amount of filler must be added to overcome the percolation threshold, the critical concentration of filler at which the polymer will conduct an electrical current. Beyond this threshold conductivity increases markedly as additional electrically conductive filler is added. It is believed that at the percolation threshold, uninterrupted chains of conducting particles first appear in the system. The addition of still greater amounts of electrically conductive filler produces a correspondingly higher number of uninterrupted chains and this results in still higher levels of conductivity.
Electrically conductive polymer systems are prized as materials for electromagnetic shielding in electronics applications and as materials used in the fabrication of structures to which paint may be applied using electrostatic painting techniques. A variety of electrically conductive fillers, such as carbon fibers, carbon fibrils and carbon black have been employed to impart electrical conductivity to otherwise insulating polymeric materials. The use of such fillers may however degrade other important physical characteristics of the material such as its impact strength. In addition, certain fillers such as carbon fibrils are high cost materials. Some electrically conductive fillers have a more pronounced negative effect on certain material""s physical properties than others, but nearly all polymer systems incorporating them suffer a degradation of impact strength, or certain other physical property not related to conductivity, relative to the unfilled polymer systems. In many instances, the desired level of electrical conductivity cannot be obtained without sacrificing at least some part of the material""s inherent impact strength. Therefore, it would be desirable to maximize the electrical conductivity enhancing effect of the conductive filler while minimizing the resultant loss in impact properties.
The instant invention is based upon the discovery that certain organic compounds function a s conductivity enhancing agents in organic conductive composite compositions, and that the inclusion of one or more of these conductivity enhancing agents reduces the amount of conductive filler required in order to achieve a given level of electrical conductivity relative to that required in the absence of the conductivity enhancing agent. The instant invention overcomes the limitations of earlier conductive composite polymer systems in that high levels of electrical conductivity can be achieved at reduced concentrations of electrically conductive filler relative to compositions lacking the conductivity enhancing agents. In this way, the present invention reduces the amount of electrically conductive filler required, thereby reducing the cost of the polymer system.
The present invention is directed to organic conductive materials comprising a conductivity enhancing agent, said organic conductive materials having improved conductivity relative to materials lacking said conductivity enhancing agent. One aspect of the invention, therefore, is an electrically conductive polymer composite composition comprising:
(A) an organic polymer matrix;
(B) an electrically conductive filler; and
(C) a conductivity enhancing agent selected from the group consisting of salts of carboxylic acids, salts of thiocarboxylic acids, salts of dithiocarboxylic acids, salts of sulfonic acids, salts of sulfinic acids, salts of phosphonic acids, salts of phosphinic acids, and mixtures thereof.
The invention further relates to methods of preparing electrically conductive polymer composite materials, to methods of enhancing the conductivity of electrically conductive polymer composite materials and articles prepared from these materials.